1. Field of the Invention
The present invention is directed to a pulse sequence in the form of a method which permits the fast calculation of images of the fat and water distribution in an examination subject on the basis of nuclear magnetic resonance.
2. Description of the Prior Art
A separate calculation of the fat and water distribution of an examination subject is often of significant diagnostic value in nuclear magnetic resonance tomography. It is known to produce separate images for the fat and water distribution to make use of the fact that the water-bonded protons in the spectrum of the magnetic resonance have a chemical shift compared to fat (methylene)-bonded protons.
A method for producing only water and fat images is described in the article .sup.1 H NMR Chemical Shift Selective (CHESS) Imaging," Haase et al., Phys. Med. Biol., 1985, Vol. 30, No. 4, pp. 341-344. In this known technique, the signal component which is not desired is first excited by a selective 90.degree. RF-pulse. Subsequently, the magnetization is destroyed by a spoiler gradient. The spin system thus remains in a status in which no resultant magnetization of the undesired component is present, whereas the Z-magnetization of the desired component remains uninfluenced. The desired component can then be read out with a standard imaging sequence, i.e., excitation with subsequent locally selective signal evaluation.
A method for the separate production of fat and water images is also described in the article "Fast Chemical Shift Imaging: Application on 3D Acquisition," Deimling et al., 6th SMRM Congress, New York, Abstract p. 447. In this method, a spectrally selective excitation pulse is achieved by means of a square-wave pulse. The frequency spectrum of a square-wave pulse is established by a sine function. The frequency spectrum can then be selected by the duration of the excitation pulse such that it has a maximum for one spectral component (for example, water) and has a minimum for a second spectral component (for example, fat). The fat or lipid spectral component is then not influenced by the square-wave excitation pulse. After a selective excitation with a 90.degree. pulse, the water component is dephased by a spoiler gradient. A three-dimensional data set can be calculated on the basis of a subsequent pulse sequence, this data set then only representing fat components. An image of only the fat distribution can thus be acquired. An image of the water distribution can be correspondingly achieved by shifting the frequency.
The design of spectrally selective RF excitation pulses is known from the article by Hore, appearing in the Journal of Magnetic Resonance, Vol. 54, 539-542 (1983). As described therein, each RF excitation pulse consists of individual pulses having the same spacing. Among other things, RF excitation pulses which are so-called "1331 pulses" are proposed, i.e., four successive individual pulses having the relative lengths of 1,3,3, and 1, with all of the pulses having the same phase. Further, pulse sequence are proposed in the article in which individual pulses have a phase shift of 180.degree., for example a pulse sequence of 1331, the bar over the number indicating a 180.degree. phase shift.
The article "A New Steady-State Imaging Sequence for Simultaneous Acquisition of Two MR Images with Clearly Different Contrasts," Bruder et al., Mag. Res. in Med., Vol. 7, pp. 35-42 (1988), discloses a pulse sequence wherein two MR images having noticeably different contrasts are simultaneously supplied. For producing these two images, use is made of the fact that, given a rapid succession of RF pulses, two signals having different T.sub.2 contrasts arise between the pulses. A slice of the examination subject is first excited. This is followed by a negative gradient in the x-direction which dephases the nuclear spins in the x-direction, and is followed by a phase-coding gradient in the y-direction. The nuclear spins are then again re-phased by a first, positive gradient in the x-direction, and the resulting signal is read out under the influence of this gradient. A second signal is read out under a second, positive gradient in the x-direction. A second negative gradient in the x-direction and a second phase-coding gradient follow, the latter gradients being designed so that, viewed over the entire sequence, all spins are rephased, so that a steady state condition can be established.
Two completely separate measuring events are always required in the aforementioned methods for the separate production of images of the fat and water distribution of an examination subject. This means that the measuring time is doubled, which does not take into account the preparation time out for each measuring event. Due to movements of the patient between measuring events, however, a correlation between the two measuring events is difficult.